Electrical energy is useful only when current remains on its intended conducting path. Excessive current can overheat wires, while damaged insulation or a faulty metal case can allow current to pass through a person.
Main electrical hazards
Overloading occurs when connected devices draw more current than a circuit can safely carry.
A short circuit forms an unintended path of very low resistance, producing a large current.
Electric shock occurs when the body becomes part of a circuit; its severity depends on current, path through the body and contact time.
Heating in wiring increases rapidly with current because the energy converted to heat is proportional to I².
Key equation
oxedI= racVR
I
current (A)
V
potential difference (V)
R
resistance (Ω)
Key equation
oxedWheat=I2Rt
Fuse
A fuse contains a thin wire designed to melt when current remains above its rated value. Melting opens the circuit and stops further heating. A replacement must have the specified rating; using an unsuitable conductor removes this protection.
Circuit breaker
A circuit breaker is an automatic switch. It opens the circuit during an overload or short circuit. After the fault has been found and removed, an appropriate breaker can be reset.
Earthing and insulation
The earth wire connects an exposed metal case to ground through a low-resistance path. If a live conductor touches the case, a large fault current flows through the earth path, helping the fuse or circuit breaker disconnect the supply. Insulation prevents contact with live conductors.
Quick exam check
State the fault: overload, short circuit, damaged insulation or live-case contact.
Explain the current path and why the current becomes dangerous.
Name the protective device and describe how it opens the circuit.
Never describe the earth wire as carrying normal operating current.
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Exam preparation
Exam Focus
Important points
Short circuit and overload
Fuse and circuit breaker
Protective earthing
I²R heating
Common mistakes
Saying that an earth wire carries the normal appliance current.
Saying that a fuse reduces current without opening the circuit.
Treating an overload and a short circuit as identical faults.
Choosing a fuse only from appliance power without first finding normal current.
Suggesting unsafe practical work on a live mains circuit.
Quick revision
Revision Zone
Review the key facts, formulas and checks before you move on.
Electrical Safety
Sticky Notes — Quick Revision
1.
Large current
Low resistance at fixed voltage produces a large current: I=VR.
2.
Fuse
A fuse melts and opens the circuit when current exceeds its safe rating.
3.
Circuit breaker
A circuit breaker trips automatically and may be reset after the fault is removed.
4.
Earth wire
The earth wire carries fault current through a low-resistance path to ground.
